x86/nospec: Simplify alternative_msr_write()

The macro is not type safe and I did look for why that "g" constraint for
the asm doesn't work: it's because the asm is more fundamentally wrong.

It does

        movl %[val], %%eax

but "val" isn't a 32-bit value, so then gcc will pass it in a register, 
and generate code like

        movl %rsi, %eax

and gas will complain about a nonsensical 'mov' instruction (it's moving a 
64-bit register to a 32-bit one).

Passing it through memory will just hide the real bug - gcc still thinks 
the memory location is 64-bit, but the "movl" will only load the first 32 
bits and it all happens to work because x86 is little-endian.

Convert it to a type safe inline function with a little trick which hands
the feature into the ALTERNATIVE macro.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/nospec-branch.h

@@ -241,15 +241,16 @@ static inline void vmexit_fill_RSB(void)
 #endif
 }
 
-#define alternative_msr_write(_msr, _val, _feature)		\
-	asm volatile(ALTERNATIVE("",				\
-				 "movl %[msr], %%ecx\n\t"	\
-				 "movl %[val], %%eax\n\t"	\
-				 "movl $0, %%edx\n\t"		\
-				 "wrmsr",			\
-				 _feature)			\
-		     : : [msr] "i" (_msr), [val] "i" (_val)	\
-		     : "eax", "ecx", "edx", "memory")
+static __always_inline
+void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
+{
+	asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
+		: : "c" (msr),
+		    "a" (val),
+		    "d" (val >> 32),
+		    [feature] "i" (feature)
+		: "memory");
+}
 
 static inline void indirect_branch_prediction_barrier(void)
 {